The invention described herein relates to digital electrophotographic imaging systems and in particular to selenium based photoreceptors used for digital as well as conventional dry powder imaging.
The prior art includes the following:
W. D. Fender, "Quantification of the Xeroradiographic Discharge Curve," SPIE Vol. 70, 1975, 364. Chlorine and arsenic doped amorphous selenium photoreceptors are disclosed having a thickness of 120 to 300 microns. The x-ray photogeneration constant and relationship of charging potential to photogenerated charge signal are measured and quantified. In reference to this invention, the paper shows that the photogenerated charge signal is directly proportional to the internal field or charging potential (for a given selenium thickness) that the photoreceptor can sustain without exhibiting excessive artifact levels.
W. Hillen et al., "Imaging Performance of a Selenium Based Detector for High-Resolution Radiography," SPIE Med. Imaging III, 1989 and "A Selenium Based Detector System for Digital Slot Radiography," SPIE Vol. 914, Medical Imaging II, 1988. A selenium drum x-ray fan-scanning ganged detector readout system is described. The x-ray source slit may increase tube loading and exposure times to excessive levels. Coupling of the two scanning slits and drum synchronization over the object or patient may be awkward due to the possible interference of these various members.
D. M. Korn et al., "A Method of Electronic Readout of Electrophotographic and Electroradiographic Images," JAPE, Vol. 4, No. 4, Fall, 1978. An amorphous selenium photoconductor fashioned in a strip electrode configuration and associated electronic readout are described. The system is on an opaque substrate and is designed for front exposure and readout. The proposed device may not have the resolution capability needed for mammography due to the embedded strip configuration. Further, processes which allow high charging and prevention of artifacts and crystallization are not discussed.
U. Schiebel, "Image Quality in Selenium Based Digital Radiography," SPIE vol. 626, Medicine XIV, PACS IV, 1986. A front surface exposure, strip-electrode probe front surface reading system is described utilizing a conventional selenium receptor. No provisions are made for back surface access, minimization of crystallization nor for high charging fields and the ensuing increase in sensitivity and charge signal.
R. C. Speiser et al., "Dose Comparisons for Mammographic Systems," Med. Phys. 13(5), September/October, 1986, 667. A selenium photoreceptor is disclosed and x-ray sensitivity is discussed and compared with film screen mammographic imaging.
In U.S. Pat. No. 3,970,844 to Finn Jr. et al., an ionographic gaseous x-ray receptor scanning system is disclosed which utilizes embedded electrode strips for image readout. Ionographic systems require a large thick-walled somewhat bulky design due to the need to pressurize the ionographic gas. The embedded electrode strips may not provide the resolution needed for x-ray mammography.
In U.S. Pat. No. 4,085,327 to Swank et al., a charge readout device utilizing a receptor of transparent layers of strips in such a manner as to minimize the series capacitance in parallel with the output signal. Little is said about the photoconductive x-ray sensitive material or about how such a device would be manufactured. Again, resolution may be limited by the strip spacing.
In U.S. Pat. No. 4,126,457 to Ciuffini, a method for producing a flexible photoreceptor is disclosed wherein the photoreceptor comprises a selenium alloy layer containing a concentration gradient of arsenic. The thickness of the alloy layer deposited in the working examples was about 60 microns, a typical thickness for line copier receptors and is below the 100 to 400 micron thickness needed for x-ray imaging. The photoreceptor of Ciuffini can contain high levels of arsenic which can cause reticulation, a chronic failure mode of thermally relaxed selenium x-ray photoreceptors not having correctly configured arsenic profiles.
In U.S. Pat. No. 4,298,671 to Kassel et al., an electrophotographic recording material is disclosed comprising a layer of amorphous selenium and a layer of crystalline selenium. The device consists of an opaque electrically conductive substrate upon which is vapor deposited a thin layer of tellurium. A layer of crystalline selenium is deposited on the tellurium followed by a layer of amorphous selenium on the crystalline selenium layer. The photoreceptor of Kassel et al., is designed for monopolar transport on an opaque substrate and is not configured for image scanning from the back surface.
In U.S. Pat. No. 4,521,808 to Ong et al., an image scanning apparatus is disclosed for obtaining a radiographic image. The photoreceptor is a standard Xerox 125 selenium plate with a Mylar top surface transparent electrode added. The device x-ray images through the 0.080 inch aluminum substrate which significantly increases the radiographic patient dose and renders the unit impractical for mammography. Laser scanning is performed from the top side through the added Mylar electrode which is used for image readout.
In U.S. Pat. No. 4,770,965 to Fender et al., a state-of-the-art one hundred and fifty micron thick photoreceptor is described in Example I, while an improved thick, high-sensitivity 320 micron photoreceptor is described in Example II. The examples and claims do not, however, include a transparent substrate for digital scanning nor is a multilayered configuration having a fractionated arsenic profile disclosed, other than requiring the top surface arsenic level not exceed 2% to prevent the reticulation artifact, a catastrophic wrinkling of the top surface.
In U.S. Pat. No. 4,961,209 to Rowlands et al., an x-ray image-scanning system is disclosed which utilizes a standard flat selenium photoreceptor having a movable transparent slit sensor electrode through which a traversing light beam discharges the photoreceptor in a raster pattern after x-ray exposure. One function of the moving slit electrode is to minimize the coupling capacitance in parallel with the sensed signal charge. The disclosure does not teach a transparent substrate nor does it specify arsenic profiling at either of the interfaces to minimize artifacts, increase life, charging potential or image contrast.
In U.S. Pat. No. 5,023,661 to Fender et al., a precharging process step is described for the x-ray selenium photoreceptor used in Xeromammography which removes a critical artifact called x-ray fatigue. The interface crystalline origin of the fatigue artifact and its role in injecting spurious charge into the photoreceptor is also shown in detail. The disclosure does not teach an arsenic profile at the interface which would eliminate the microcrystallites and the fatigue artifact they produce.